Cellular communication networks are well known in the art. An example of such a network is a cellular telephone network. Another example is the use of cellular communication in conventional telephone networks, i.e. for replacing physical (wire) connections between a central office and the subscribers units in dense urban areas where wiring is not practical or too costly.
Since a limited range of frequencies is allocated for each cellular telecommunication network, methods were developed to allow simultaneous use of frequency bands within the allocated range of frequencies. This enables an increased number of users per area to use the network at the same time.
One method for improving the use of an allocated range of frequencies includes reusing frequency bands within the allocated range of frequencies within the area covered by the network. This method which is known as the "cell method", is schematically shown in FIG. 1A to which reference is now made. The network of FIG. 1A covers an area indicated by reference numeral 12. The area 12 is divided into subareas, termed "cells", such as cell 14. Each cell includes a base station, such as base station 16, typically in its center. Each base station includes at least one non-directional antenna which communicates with any telephone or local station within the area covered by the cell.
Typically, two adjacent cells, such as cells 18 and 20 or cells 22 and 24, use different frequency bands within the range of frequencies allocated to the network. A typical width of a frequency band is on the order of 1 Mhz. In the network of FIG. 1, each cell operates in one of seven frequency bands within the range of frequencies allocated for the network. The bands are denoted F1, F2, F3, F4, F5, F6 and F7.
In a cellular network the reuse factor N, i.e. the number of distinct frequency bands required for covering the desired area, depends mainly on the capability of the antenna of a base station to cope with interference generated by the network users in other cells using the same frequency band. This capability is a function of the distance D between the interfering user unit and the signal carrier antenna. In a network, such as the network 10, which employs a reuse factor of 7, interference to an antennae in any cell, such as in cell 24, may come from any of the six closest cells which operate on the same frequency band, such as from users in cell 28.
The reuse factor also depends on the Carrier to Interference (C/I) ratio which measures the ratio between the carrier signal of an antennae and the interfering noise. For most modern networks, it is desired that the C/I ratio will be 7 dB-15 dB and preferably larger than 15 db to minimize interferences. Therefore, for a state of the art network which employs non-directional antennae, such as the network 10, a reuse factor of 7, as indicated by the notations F1-F7, is required in order to achieve a C/I ratio of 18.6 dB which meets the desired C/I value of 15 dB.
One way to reduce the reuse factor, in order to use a smaller range of frequencies, is to employ directional antennae in each one of the cells instead of the non-directional antennae of the network 10. A typical prior art cellular network which employs directional antennae is shown in FIG. 1B to which reference is now made.
In the network of FIG. 1B, referenced generally 50, three directional antennae are employed in each cell. Each antenna is directed to a different direction and is using a different frequency band of the three frequency bands denoted F1, F2 and F3.
In prior art networks which employ directional antennae in adjacent as well as in distant cells, antennae which are directed to the same direction employ a similar frequency band. For example, in the network 50, the antennae 52, 56 and 60 which employ the F2 band are directed in the same direction and cover the same area of the cells, 54, 58 and 62, respectively, i.e. the areas in the upper right side of each cell denoted 64, 66 and 68, respectively.
In the network 50, an improved C/I ratio relative to a similar network which employs non-directional antennae is achieved since interferences come to any antennae from two adjacent cells and not from six cells as in the network 10 as described in more detail with reference to FIG. 1C.
Reference is now made to FIG. 1C which is a schematic illustration of part of the network 50 in an operation mode. For clarity of presentation only, four cells in FIG. 3, cells 54, 58, 62 and 90 are shown in detail whereas the other cells are outlined only.
For each of the cells 54, 58, 62 and 90, there is shown a unit which communicates with one of the directional antennae in each cell, the antennae which employs the F3 frequency band. In the cell 54, unit 84 communicates with the antenna which covers the area 164, in the cell 58, unit 82 communicates with the antenna covering the area 66, in the cell 62, unit 86 communicates with the antenna covering the area 68 and in the cell 90 and unit 96 communicates with the antenna covering the area 92.
A characteristic of prior art networks, such as the network 50 is that interferences to any antennae come from two adjacent cell and also from more distant cells. For example, interferences to the antenna which employs the F3 frequency band in the cell 54 come from the units 82 and 86 of the cells 58 and 62, respectively, and to much less extent from the unit 90 of the cell 92. This is because all the antennae of the cells 53, 58, 62 and 92 which employ the F3 frequency band are directed in the same general direction as indicated by arrows 74, 82, 86 and 94, respectively.
For the network 50 the C/I ratio with comparable technology to that of the network 10 is 19.1 dB. For a network like network 50, with a reuse factor of 7, i.e. which employs altogether 21 frequency bands, three per each cell and with a reuse factor of 7, the C/I ratio is 23.4 dB.